Nox purging system and method of reactivating deteriorated catalyst therein

ABSTRACT

A NOx purging system using a direct reduction type NOx catalyst, wherein sulfur is purged while avoiding the occurrence of secondary sulfur poisoning, taking advantage of capability of recovery from deterioration caused by sulfur poisoning at exhaust gas temperature exhibited at normal operating zone, so that the influence of sulfur poisoning can be eliminated and NOx is efficiently purged; and a method of reactivating deteriorated catalyst therein. In particular, a NOx purging system ( 10 ) comprising exhaust gas passage ( 2 ) and, arranged therein, direct reduction type NOx catalyst ( 3 ), which NOx purging system ( 10 ) is fitted with first sulfur purge control means ( 222 ) for performing such first sulfur purge operation that when exhaust gas temperature (Tg) becomes higher than given set temperature (T1) during normal operation, not only is the oxygen concentration of exhaust gas decreased but also the exhaust temperature (Tg) is raised to not below sulfur purge temperature (Tr).

TECHNICAL FIELD

The present invention relates to an exhaust gas NOx purging system ofreducing and purging NOx (nitrogen oxides) in the exhaust gas of aninternal combustion engine and a method of reactivating a deterioratedcatalyst therein. Particularly, it relates to a means and method forreactivating a direct reduction type NOx catalyst used for the NOxpurging system from the deteriorated state due to sulfur poisoning.

BACKGROUND ART

Various studies and proposals have been offered regarding acatalyst-type exhaust gas purifying system for eliminating NOx from theexhaust gas of an internal combustion engine of a vehicle or astationary internal combustion engine by reducing NOx. Particularly, aNOx reduction catalyst or a three-way-catalyst is in use to purify theexhaust gas of vehicles.

One of the studies and proposals shows an exhaust gas purifying systemfor internal combustion engine disclosed in the official gazette of theJapanese Patent Laid-Open No. 2000-274279 and the like. In this system,a NOx occlusion reduction catalyst is arranged in an exhaust gas passageof an engine. This system performs the absorbing operation of making theNOx occlusion reduction catalyst absorb NOx during the air/fuel ratio ofan incoming exhaust gas is lean. When the NOx absorbing capabilityalmost levels off, the regenerating operation is performed to make theair/fuel ratio of the exhaust gas close to a theoretical air/fuel ratioor rich and to make an oxygen concentration of the incoming exhaust gaslow, and thereby releasing the absorbed NOx, and reducing the releasedNOx by a laid-in precious metal catalyst.

The NOx occlusion reduction catalyst supports a precious metal catalystsuch as platinum (Pt) and alkaline earth such as barium (Ba) on acatalyst support. NO in the exhaust gas is oxidized by a catalystactivity of platinum and changed to NO₂ under ahigh-oxygen-concentration atmosphere. NO₂ is diffused in the catalyst inthe form of NO₃ ⁻, and absorbed in the form of nitrate.

Moreover, when the air/fuel ratio becomes rich and the oxygenconcentration lowers, NO₃ ⁻ is released in the form of NO₂. And the NO₂is reduced to N₂ in accordance with the catalyst activity of platinum bythe reducer such as unburned HC, CO, and H₂ contained in the exhaustgas. It is possible to prevent NOx from being released to theatmospheric air in accordance with the above reducing effect.

However, in the case of the exhaust gas purifying system using the NOxocclusion reduction catalyst, an extremely large quantity of NOx isreleased in a short time when the NOx occlusion reduction catalyst isregenerated. Therefore, it is necessary to reduce the NOx by a preciousmetal catalyst. However, even if supplying a proper quantity ofreducers, it is difficult to reduce the whole of NOx to N₂ by securelybringing the whole of NOx into contact with the reducers and theprecious metal catalyst. Then some of NOx leaks. Therefore, there is aproblem that a decrease of NOx is limited.

Moreover, because a catalyst function is deteriorated by the sulfurcontained in the fuel of a diesel engine, there is a problem of sulfurpoisoning that it is difficult to keep a rate of NOx purge high for along time. Therefore, the exhaust gas purifying system in the officialgazette of the Japanese Patent Laid-Open No. 2000-274279 performs thedeterioration judgment that a NOx occlusion reduction catalyst isdeteriorated when the NOx concentration at the end of release of NOx isequal to or higher than a predetermined reference value for thedeterioration in accordance with the characteristic of an occlusioncatalyst which is based on the storage and the release of a largequantity of NOx by absorbing substance.

For the sulfur purge to reactivate the catalyst from the deterioratedstate due to the sulfur poisoning, it is necessary to raise thetemperature of the catalyst up to 650° C. In the case of a dieselengine, it is necessary to raise the exhaust gas temperature to 600° C.or higher in order to raise the catalyst temperature to 650° C. orhigher. However, even if performing the exhaust gas temperature raisingcontrol such as intake throttling or rich burning, it is actuallydifficult to raise the catalyst temperature up to 650° C. by onlycontrolling the engine.

However, separately from the NOx occlusion reduction catalyst, acatalyst for directly reducing NOx (hereafter referred to as directreduction type NOx catalyst) is disclosed in the patent applications toRepublic of Finland No. 19992481 and 20000617.

The direct reduction type NOx catalyst is obtained by making a catalystsupport T such as âtype zeolite support a metal M such as rhodium (Rh)or palladium (Pd) which is a catalyst component as shown in FIGS. 7 and8. As shown in FIG. 7, in the case of a high oxygen concentrationatmosphere such as the lean state exhaust gas of an internal combustionengine such as a diesel engine in which the air/fuel ratio of theexhaust gas is lean, the catalyst component contacts with NOx andreduces NOx to N₂. At the same time, it is oxidized to become metaloxide MOx such as rhodium oxide. After all of the metal M is oxidized,the capability of NOx reduction disappears. Therefore, it is necessaryto regenerate the metal M when it is oxidized to a certain extent.

As shown in FIG. 8, the above regeneration is performed by setting theoxygen concentration of the exhaust gas to almost equal to 0% as theair/fuel ratio is in a theoretical air/fuel ratio or a rich state, bybringing the metal oxide MOx such as rhodium oxide into contact withreducer such as unburned HC, CO, and H₂ in a reduction atmosphere toreduce the metal oxide MOx and by returning the metal oxide MOx to itsoriginal metal M.

In the case of the direct reduction type NOx catalyst, the reaction forreducing the metal oxide MOx is quickly performed even at a lowtemperature (e.g. 200° C. or higher) compared to the case of othercatalyst and moreover, there is an advantage that the problem of sulfurpoisoning is small.

Moreover, cerium (Ce) is blended. This cerium contributes for decreasingthe oxidation of the metal M and for holding the capability of reductionof NOx. And a three-way-catalyst is set to a lower layer to acceleratethe reaction of reduction and oxidation, particularly the reaction ofreducing NOx in a rich state. Moreover, iron (Fe) is added to a catalystsupport in order to improve the rate of NOx purge.

However, though sulfur poisoning is small compared to the case of theNOx occlusion reduction catalyst sulfur poisoning is slowly progressedby the sulfur in a fuel and the deterioration of the catalyst isprogressed. That is, because the sulfur contained in the exhaust gas isabsorbed as SO₂ in the iron added to the catalyst support, primarysulfur poisoning occurs in which the improvement of the purgeperformance of NOx due to the iron is inhibited. Moreover, in anoxidizing atmosphere containing no reducer at a constant temperature,SO₂ discharged from iron is changed to SO₃ and the SO₃ is combined withcerium. Therefore, secondary sulfur poisoning occurs and the rate of NOxpurge is decreased. In this secondary sulfur poisoning, the contributionof the cerium to holding the capability of the NOx reduction is lowered.

However, in the case of the direct reduction type NOx catalyst, theexhaust gas temperature necessary to reactivate the deterioratedcatalyst due to the sulfur poisoning is approx. 400° C. This temperatureis low comparing to the NOx occlusion reduction catalyst. Therefore, itis easily possible to achieve the temperature even in a normal drivingstate.

FIG. 6 shows a result of experimenting the sulfur poisoning state by asimulated gas system. FIG. 6 shows a result of performing an experimentof setting an exhaust gas temperature to a specific temperature of 150°C. to 400° C., repeatedly generating a lean state gas and a rich stategas, and then measuring the rate of NOx purge.

According to FIG. 6, a high purge rate is shown in the initial state (acontinuous line A with a symbol ▪). However, it is found that the purgerate is deteriorated to approx. 63% when setting an exhaust gastemperature to 300° C. and 22.5 hr elapses without performingreactivation of a deteriorated catalyst through sulfur purge (points Bwith a symbol ⋄). Moreover, it is found that sulfur poisoning progressesand the purge rate is deteriorated by approx. 20% to 30% compared to thecase of the initial state (a continuous fine A with a symbol ▪) whensetting the exhaust gas temperature to 450° C. and simply repeating thelean state and rich state of NOx reduction (a continuous line C with asymbol □).

When the deterioration due to the sulfur poisoning progresses in thedirect reduction type NOx catalyst, the rate of NOx purge is decreasedbecause the capability of reducing NOx to N₂ is decreased even in thecase of an atmosphere in which the air/fuel ratio of the exhaust gas isin the lean state and the oxygen concentration is high and moreover, theNOx reduction capability is immediately decreased to a value close to alimit. Therefore, because it is necessary to frequently performregeneration through rich burning, fuel efficiency decreases.

Therefore, in the case of the direct reduction type NOx catalyst, it isnecessary to perform not only the regeneration of reducing the metaloxide MOx to the metal M by bringing into contact with reducer in areducing atmosphere but also the catalyst reactivation by the sulfurpurge. Monitoring the progress state of deterioration due to the sulfurpoisoning, when the deterioration is progressed to a certain extent, theexhaust gas temperature is setting to approx. 400° C. in a low oxygenconcentration state to remove sulfur and the catalyst reactivation isforcibly performed.

However, a state in which the exhaust gas temperature becomes a sulfurpurge temperature of approx. 400° C. occurs at a high frequency even bythe normal lean state operation. And when the oxygen concentration iscomparatively high and the exhaust gas temperature reaches the sulfurpurge temperature, sulfur is separated from iron and primary sulfurpoisoning is eliminated. However, a problem occurs that the sulfurcombines with oxygen contained in the exhaust gas to produce SO₃ and theSO₃ combines with cerium to cause new sulfur poisoning, that is,secondary sulfur poisoning which is difficult to eliminate.

However, in the case of the experiment of the simulated gas system shownin FIG. 6, before raising the exhaust gas temperature to 400° C., therich operation is carried to perform sulfur purge and after thisreactivating a deteriorated catalyst, the NOx regeneration experiment iscarried to raise the exhaust gas temperature to 400° C. and to repeatthe lean state and rich state. Then, it is found that the rate of NOxpurge at 400° C. is not deteriorated and even after a durability test,the purge rate is almost returned to a state close to the initial state(a continuous line A with a symbol ▪) as shown by a dotted line with asymbol Δ. From this fact, it is found that the temperature reaches 400°C. after performing the sulfur purge but secondary sulfur poisoning doesnot occur even if the lean state is set.

SUMMARY OF THE INVENTION

The present invention is made to solve the above problems in accordancewith the above knowledge and its object is to provide a NOx purgingsystem using a direct reduction type NOx catalyst for purging NOxcontained in an exhaust gas and a method of reactivating a deterioratedcatalyst, in which exploiting the characteristics that a reactivatingcatalyst deteriorated by sulfur poisoning is possible at an exhaust gastemperature of a normal operating range, a rich control operation isperformed to raise the temperature of the direct reduction type NOxcatalyst to the sulfur purge temperature (approximately 400° C.) orhigher under a low-oxygen condition when the exhaust gas temperaturereaches to a predetermined temperature (350° C. to 400° C.) or higherduring the normal operation of an engine so that NOx is removedefficiently through the sulfur purge preventing secondary sulfurpoisoning by excluding the influence of sulfur poisoning.

A NOx purging system for achieving the above object and a method ofreactivating a deteriorated catalyst therein are constituted asdescribed below.

1) The NOx purging system constituted by arranging a direct reductiontype NOx catalyst in an exhaust gas passage in which a catalystcomponent reduces NOx to nitrogen and is also oxidized when an oxygenconcentration of the exhaust gas of an engine is high and the catalystcomponent is reduced when the oxygen concentration of the exhaust gaslowers, comprising; a first sulfur purge control means which performs afirst sulfur purge operation for lowering the oxygen concentration ofthe exhaust gas and also raising an exhaust gas temperature to a sulfurpurge temperature or higher when the exhaust gas temperature becomeshigher than a predetermined temperature during the normal operation.

2) Moreover, the NOx purging system above comprises; a second sulfurpurge control means which performs a second sulfur purge operation forlowering the oxygen concentration of the exhaust gas and also raisingthe exhaust gas temperature to the sulfur purge temperature or higherindependently of the exhaust gas temperature, and a sulfur purge startjudgment means for judging that the present time is a start time of thefirst sulfur purge operation when a deterioration index value indicatinga progress state of catalyst deterioration due to sulfur poisoningranges between a predetermined first reference value and a predeterminedsecond reference value, and judging that the present time is a starttime of the second sulfur purge operation when the deterioration indexvalue exceeds the predetermined second reference value.

The method of reactivating a deteriorated catalyst of the NOx purgingsystem is constituted as described below.

1) The method of reactivating a deteriorated catalyst for a NOx purgingsystem constituted by arranging a direct reduction type NOx catalyst inan exhaust gas passage in which a catalyst component reduces NOx tonitrogen and is also oxidized when an oxygen concentration of theexhaust gas is high and the catalyst component is reduced when theoxygen concentration of the exhaust gas lowers, comprising steps of;carrying a first sulfur purge operation control to perform a firstsulfur purge operation for lowering the oxygen concentration of theexhaust gas and also for raising the exhaust gas temperature to a sulfurpurge temperature or higher when the exhaust gas temperature becomesmore than a predetermined temperature during the normal operation.

2) Moreover, the method of reactivating the deteriorated catalyst forthe NOx purging system above-mentioned comprises steps of; performingthe first sulfur purge operation when a deterioration index valueindicating a progress state of a catalyst deterioration due to sulfurpoisoning ranges between a predetermined first reference value and apredetermined second reference value, and performing a second sulfurpurge operation for lowering the oxygen concentration of the exhaust gasand also for raising the exhaust gas temperature to the sulfur purgetemperature or higher when the deterioration index value exceeds thepredetermined second reference value independently of the exhaust gastemperature.

3) Furthermore, the method of reactivating the deteriorated catalyst forthe NOx purging system above comprises step of; calculating thedeterioration index value indicating a progress state of catalystdeterioration due to sulfur poisoning in accordance with a fuelconsumption and a sulfur concentration of fuel.

In this case, an accumulated value of the quantity of deposited sulfurcalculated in accordance with the fuel consumption and the sulfurconcentration of fuel is used as the deterioration index value.

4) Then, in at least either of the first sulfur purge operation and thesecond sulfur purge operation, when the difference between the sum totalof discharged sulfur quantity calculated by collating a discharged gasquantity and exhaust gas temperature with discharged sulfur quantity mapdata previously input and the accumulated sulfur quantity calculated inaccordance with the fuel consumption and the sulfur concentration offuel becomes not more than a predetermined third reference value, thesulfur purge operation is terminated.

5) Or, in at least either of the first sulfur purge operation and thesecond sulfur purge operation, when a sulfur purge operation timeelapses which is calculated by collating an accumulated sulfur quantitycalculated in accordance with the fuel consumption and the sulfurconcentration of fuel with sulfur purge operation time map datapreviously input in accordance with an exhaust gas quantity and theexhaust gas temperature, the sulfur purge operation is terminated.

It is possible to constitute the direct reduction type NOx catalyst bymaking a support such as âtype zeolite support a special metal such asrhodium (Rh) or palladium (Pd) which is a catalyst component. Moreover,it is possible to form the direct reduction type NOx catalyst, blendingcerium (Ce) in order to contribute to holding the NOx reductioncapability by decreasing the oxidation action of a metal M of thecatalyst component, and arranging a three-way-catalyst having platinumor the like on a lower layer in order to accelerate a redox reaction,particularly the reaction of reducing NOx discharged in a rich state, oradding iron (Fe) to the support in order to improve the rate of NOxpurge.

A catalyst in which a catalyst component reduces NOx to N₂ and thecatalyst component is oxidized when the oxygen concentration of theexhaust gas is high, and the catalyst component is reduced when theoxygen concentration of the exhaust gas is decreased, is referred to as“direct reduction type NOx catalyst” in this case in order todistinguish the catalyst used in this case from catalysts used in otherprior arts.

Moreover, the normal operation denotes an operation performed at atorque or an engine speed required for an engine when an operation forregenerating a catalyst or an operation for reactivating a deterioratedcatalyst is not performed. In the case of the normal operation, NOxcontained in the exhaust gas is directly reduced to N₂ through thedirect reduction type NOx catalyst.

Moreover, the first sulfur purge operation can be performed through arich spike control according to an intake air quantity control such asan intake air throttling, a fuel injection control such as apost-injection, or an EGR control. Furthermore, the predeterminedtemperature is one of numerical values or map data obtained throughexperiments and the like, that is, a predetermined value indicating atemperature ranging between 350° C. and 400° C.

It is also allowed to perform the control in accordance with thecatalyst temperature instead of the exhaust gas temperature.

According to the NOx purging system and the method of reactivating thedeteriorated catalyst therein, in the case of the NOx purging systemusing the direct reduction type NOx catalyst for purging NOx in theexhaust gas, since the deteriorated catalyst reactivation against sulfurpoisoning can be performed at an exhaust gas temperature of the normaloperation it is possible to exclude an influence of sulfur poisoning andefficiently purge NOx by performing the rich spike for decreasing theoxygen concentration of the exhaust gas and for raising exhaust gastemperature to the sulfur purge temperature (approx. 400° C.) or higherunder low-oxygen conditions when the exhaust gas temperature becomeshigher than a predetermined temperature during the normal engineoperation, thereby performing the deteriorated catalyst reactivationaccording to the sulfur purge while preventing a secondary sulfurpoisoning.

Moreover, in the first sulfur purge operation, because the rich spike isperformed only when the exhaust gas temperature rises, the fuelconsumption for raising the exhaust gas temperature is decreased and thefuel efficiency can be prevented from decreasing.

That is, the first sulfur purge operation for performing the sulfurpurge when the exhaust gas temperature rises higher during the normaloperation makes it possible to perform the deteriorated catalystreactivation according to the sulfur purge while preventing secondarysulfur poisoning, by setting the exhaust gas temperature not less thanthe sulfur purge temperature under low-oxygen conditions while savingfuel consumption for raising the exhaust gas temperature.

Then, by the first sulfur purge operation, after sulfur is deposited onthe catalyst to a certain extent, it is possible to perform a sulfurpurge before the exhaust gas temperature becomes not less than thesulfur purge temperature. Therefore, it is possible to avoid that theexhaust gas temperature becomes not less than the sulfur purgetemperature while keeping the lean state of the normal operation. Then,it is possible to prevent secondary sulfur poisoning and moreoverperform the deteriorated catalyst reactivation at high frequency.

And, when the catalyst deterioration progresses though the exhaust gastemperature does not become higher than a predetermined temperatureduring the normal operation, it is possible to steadily perform thedeteriorated catalyst reactivation in accordance with the second sulfurpurge operation before the influence of catalyst deterioration isintensified.

Furthermore, by properly using the first sulfur purge operation and thesecond sulfur purge operation, in the case the progress of the catalystdeterioration is insignificant, it is possible to restrain thedecreasing of fuel efficiency caused by raising the exhaust gastemperature and moreover to perform the deteriorated catalystreactivation at high frequency, by performing the deteriorated catalystreactivation in accordance with the first sulfur purge operation whichis performed only when the exhaust gas temperature becomes higher than apredetermined temperature during the normal operation.

And it is possible to avoid unnecessary sulfur purge operations makingno contribution to the deteriorated catalyst reactivation and torestrain the decreasing of fuel efficiency by the method of calculationof the deterioration index value and the method of judgment of sulfurpurge operation end in the method of reactivating a deterioratedcatalyst.

Therefore, it is possible to restrain the deterioration of the directreduction type NOx catalyst due to sulfur poisoning, particularly due tosecondary sulfur poisoning of cerium or the like. Moreover, since thesulfur purge can be performed at high frequency, it is not necessary toperform the rich spike operation for a long time, and the torquefluctuation often triggered by this operation is eliminated. Therefore,it is possible to eliminate the mal-drivability following thedeteriorated catalyst reactivation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a configuration of a NOx purgingsystem of an embodiment of the present invention;

FIG. 2 is an illustration showing a configuration of a NOx purgingsystem control means of an embodiment of the present invention;

FIG. 3 is a flowchart showing an example of a NOx purging system controlflow of an embodiment of the present invention;

FIG. 4 is a flowchart showing an example of the catalyst regeneratingcontrol flow in FIG. 3;

FIG. 5 is a flowchart showing an example of the deteriorated catalystreactivation control flow in FIG. 3;

FIG. 6 is an illustration showing a relation between an exhaust gastemperature and a purge rate of a direct reduction type NOx catalystobtained through an experiment in a simulated gas system;

FIG. 7 is a schematic view showing a reaction of a direct reduction typeNOx catalyst at a high oxygen concentration; and

FIG. 8 is a schematic view showing a reaction of a direct reduction typeNOx catalyst at a low oxygen concentration state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of a NOx purging system and a method of reactivating adeteriorated catalyst therein according to the present invention aredescribed below by referring to the accompanying drawings.

First, the NOx purging system is described. As shown in FIG. 1, the NOxpurging system 10 is provided with a direct reduction type NOx catalyst3 arranged in an exhaust gas passage (a passage for exhaust gas) 2 of anengine body 1.

As shown in FIGS. 7 and 8, the direct reduction type NOx catalyst 3 isconstituted by making a support T such as âtype zeolite support aspecial metal M such as rhodium (Rh) or palladium (Pd). Moreover, cerium(Ce) is blended for reducing an oxidization of the metal M andcontributing to holding of a NOx reduction capability, athree-way-catalyst having platinum or the like is arranged to a lowerlayer so as to accelerate a redox reaction, and iron (Fe) is added to asupport in order to improve a rate of NOx purge.

Then, the direct reduction type NOx catalyst 3 has the characteristicthat it reduces NOx to N₂ contacting with NOx in an atmosphere of a highoxygen concentration like an exhaust gas of an internal combustionengine such as a diesel engine in which the air/fuel ratio is lean andthe metal M itself is oxidized to become metal oxide MOx such as rhodiumoxide (RhOx) as shown in FIG. 7, and that the metal oxide MOx is reducedto become its original metal M such as rhodium by contacting withreducers such as unburned HC, CO, and H₂ in the case of a reductionatmosphere in which an oxygen concentration of the exhaust gas is lowalmost equal to 0% as the air/fuel ratio is equal to a theoreticalair/fuel ratio or in a rich state as shown in FIG. 8,

Moreover, an operating state detector 5 is set which is constituted by atorque sensor and an engine speed sensor for detecting an operatingstate of an engine, mainly a torque Q and an engine speed Ne.Furthermore, in the exhaust gas passage 2, an air/fuel ratio sensor 6for detecting an air/fuel ratio Af is set upstream of the directreduction type NOx catalyst 3, an exhaust gas temperature sensor 7 fordetecting an exhaust gas temperature Tg is set upstream of the directreduction type NOx catalyst 3, and moreover a NOx sensor 8 for detectinga NOx concentration Cnox is set downstream of the NOx catalyst 3.

Then, a controller 4 referred to as an engine control unit (ECU) forperforming the general control of an engine such as fuel injectioncontrol by using the torque (load) Q and engine speed Ne of the engine 1obtained from the operating state detector 5 or the like as inputs isconstituted and a NOx purging system control means for performing thecatalyst regeneration control and deteriorated catalyst reactivationcontrol of the direct reduction type NOx catalyst 3 is set to thecontroller 4.

As shown in FIG. 2, a NOx purging system control means 200 is providedwith a catalyst regeneration means 210 including a regeneration timejudgment means 211 and a regeneration control means 212 and adeteriorated catalyst reactivation means 220.

The catalyst regeneration means 210 is a means for regenerating thedirect reduction type NOx catalyst 3 in the state of low oxygenconcentration where the air/fuel ratio of the exhaust gas is in a richstate. The catalyst 3 has become the metal oxide MOx by contacting withNOx to reduce NOx to N₂ in the normal operating state of high oxygenconcentration where the air/fuel ratio of the exhaust gas is in a leanstate. The means 210 generates the exhaust gas of theoretical air/fuelratio or a rich state where oxygen concentration is almost equal to 0%by the regenerating control means 212, judging the time for performingthe regeneration by the regeneration time judgment means 211, and makesthe metal oxide MOx contact with reducers such as unburned HC, CO, andH₂ to reduce the metal oxide MOx and return it to the metal M.

The regeneration time judgment means 211 judges whether it is theregeneration time or not, by the NOx concentration Cnox of the exhaustgas downstream of the direct reduction type NOx catalyst 3 when reducingNOx, by the elapsed time when the oxygen concentration is high, or bythe estimated value of the NOx quantity reduced by the direct reductiontype NOx catalyst 3 when reducing NOx.

Moreover, the regeneration control means 212 is a means for decreasingthe oxygen concentration of the exhaust gas, that is, a means forperforming the rich spike operation with the air/fuel ratio Af of 14.7or less, which performs any one or a combination of the controls such asa fuel injection control for controlling the injection of the fuel to besupplied to the combustion chambers of an internal combustion engine, anintake air quantity control for controlling the quantity of the intakeair, and an EGR control for controlling the quantity of EGR gas in anEGR system, and performs a feedback control so that the detection valueAf is kept within a predetermined range in accordance with the detectionvalue Af of the air/fuel ratio sensor 6.

The fuel injection control includes a main injection time control forchanging time of the main fuel injection into the combustion chambers ofan engine and a post-injection control for performing a post-injectionafter a main injection and the intake air control includes an intakethrottle valve control for controlling a valve opening of anot-illustrated intake throttle valve and a turbocharger intake controlfor controlling the quantity of an intake air from a compressor of anot-illustrated turbocharger.

Moreover, the deteriorated catalyst reactivation means 220 is providedwith a sulfur purge start judgment means 221, a first sulfur purgecontrol means 222, and a second sulfur purge control means 223.

The sulfur purge start judgment means 221 judges whether to performeither of the first sulfur purge operation and the second sulfur purgeoperation. It estimates a sulfur quantity X₁ deposited on the directreduction type NOx catalyst 3 according to the fuel consumption and thesulfur concentration of the fuel. It judges to start the first sulfurpurge operation when an accumulated value Xt obtained by accumulatingthe deposited sulfur quantity X₁ is larger than a first purge startjudgment value X1 and smaller than a second purge start judgment valueX2, and judges to start the second sulfur purge operation when theaccumulated value Xt is larger than the second purge start judgmentvalue X2 and not to start any of sulfur purge operation in a case otherthan the above cases.

Moreover, the first sulfur purge control means 222 does not have toimmediately perform the sulfur purge operation by assuming that theaccumulated sulfur quantity Xt does not reach a limit X2 though Xtincreases to a certain extent. However, the means 222 performs the richspike operation for decreasing the oxygen concentration of the exhaustgas and raising an exhaust gas temperature Tg to a sulfur purgetemperature Tr (approx. 400° C.) or higher when the exhaust gastemperature Tg becomes higher than a predetermined temperature Tc (350°C. to 400° C.) during the normal operation by assuming that the sulfurpurge operation must be performed according to a necessity. And itpurges and reactivates a deteriorated catalyst preventing secondarysulfur poisoning under low-oxygen conditions. Because the first sulfurpurge operation is performed when the exhaust gas temperature Tg is kepthigh, only a small quantity of fuel is consumed to raise the exhaust gastemperature.

Furthermore, the first sulfur purge control means 222 performs sulfurpurge before the exhaust gas temperature Tg becomes not less than thesulfur purge temperature Tr after sulfur is deposited to a certainextent on the catalyst 3. Therefore, it is possible to avoid that theexhaust gas temperature Tg becomes not less than the sulfur purgetemperature Tr under the lean state of the normal operation and toprevent a secondary sulfur poisoning.

And, the second sulfur purge means 223 performs the rich spike operationfor decreasing the oxygen concentration of the exhaust gas and forciblyraising the exhaust gas temperature Tg to the sulfur purge temperatureTr or higher by assuming that a sulfur purge is immediately necessarybecause the accumulated sulfur quantity Xt reaches the limit X2,independently of the exhaust gas temperature Tg even when the exhaustgas temperature Tg is lower than a predetermined temperature Tc. And itpurges sulfur and reactivates a deteriorated catalyst preventing thesecondary sulfur poisoning under a rich state.

It is possible to perform the rich spike operation in the first andsecond purge operations in accordance with any one of the fuel injectioncontrol, an intake air control, and an EGR control or a combination ofthem similar to the case in the rich spike operation for theregenerating process.

Then, a NOx purging system control flow is described below in which NOxis purged from the exhaust gas by controlling the NOx purging system 10of above configuration by the NOx purging system control means 200. Thecontrol flow is performed in accordance with the flowcharts and the likeillustrated in FIGS. 3 to 5.

The NOx purging system control flow shown in FIG. 3 comprises thecatalyst regeneration control in step S100 and the deteriorated catalystreactivation control in step S200. It is composed as a part of a generalflow for controlling the whole of an engine, and is called from a mainsystem control flow. It is carried in parallel with an engine controlflow and thereafter the flow returns to the main engine control flow,and completed when the engine control flow is completed.

Moreover, as shown in FIG. 3, when the NOx purging system control flowstarts, the catalyst regeneration control and deteriorated catalystreactivation control are executed in parallel.

As shown by the catalyst regeneration control flow in FIG. 4, thecatalyst regeneration control performs the normal operation control forpurging NOx by the direct reduction type catalyst 3 for a given time(for example, the time corresponding to a time period for judgingwhether to perform the catalyst regeneration control) in step S110 andthen, judges in step S120 whether the direct reduction type catalyst 3is in a regenerating start time, and when it is judged to be in theregeneration start time, after the regenerating control is performed instep S130, but when it is not judged to be in the regenerating starttime, the flow directly returns to step S10 to repeat the above control.

Moreover, when the control flow is completed in such a case as acompletion of the engine operation, an interruption of the completion instep S140 occurs and the flow returns to the NOx pursing system controlflow in FIG. 3 to complete the flow.

Then, in the case of the deteriorated catalyst reactivation control, asshown by the deteriorated catalyst reactivation control flow in FIG. 5,when the flow starts, the accumulated quantity Xt of the sulfuraccumulated on the direct reduction type NOx catalyst 3 by the last-timeengine operation is read from a memory in step S11.

Then, in step S21, the normal operation control is performed for apredetermined time (for example, time corresponding to a time period forjudging whether to perform deteriorated catalyst reactivation control),and a deposited sulfur quantity Xa by the engine operation for thepredetermined time is calculated in accordance with fuel consumption anda sulfur concentration of fuel, and the deposited sulfur quantity Xa isadded to the accumulated quantity xt to make Xt a new accumulatedquantity Xt (Xt=Xt+Xa).

In next step S22, the first sulfur purge start time is judged whether itis the first sulfur purge start time by whether the accumulated quantityXt is larger than a given first purge start judgment value X1. When Xtis not larger than X1, the flow returns to step S21, assuming that it isnot in the first sulfur purge start time.

Moreover, when it is judged in step 22 that the accumulated quantity Xtis larger than the given first purge start judgment value X1, it isassumed it is in the first sulfur purge start time. However further innext step S23, the second sulfur purge start time is judged by whetherthe accumulated quantity Xt is larger than the given second purge startjudgment value X2. When Xt is smaller than X2, the flow goes to S30 toperform the first sulfur purge operation by assuming it is not in thesecond sulfur purge start time. When Xt is larger than X2, the flow goesto S40 to perform the second sulfur purge operation by assuming it is inthe second sulfur purge start time.

In the case of the first sulfur purge operation in step S30, it isjudged whether the exhaust gas temperature Tg is higher than thepredetermined temperature Tc in step S31. When the exhaust gastemperature Tg is higher than the predetermined temperature Tc, it isfurther judged in step S32 whether the present operation is the normaloperation in which other operations such as the regeneration control isnot performed.

When the exhaust gas temperature Tg is higher than the predeterminedtemperature T1 and the normal operation is performed as a result ofjudgments in both steps S31 and S32, the flow goes to the first sulfurpurge operation control in step S33. In other cases, that is, when theexhaust gas temperature Tg is lower than T1 or the regeneratingoperation is currently performed, it is not assumed to be in a state forthe first sulfur purge operation and the flow returns to S21.

In the case of the first sulfur purge operation control in step S33, thefirst sulfur purge operation is performed for a predetermined time andin step S34, a discharged sulfur quantity Xs during the first sulfurpurge is calculated by collating the exhaust gas quantity and theexhaust gas temperature Tg with discharged sulfur quantity map datapreviously input and subtracting the discharged quantity Xs from theaccumulated quantity Xt to obtain the accumulated quantity Xt afterperforming the first sulfur purge operation control in step S33.However, when the accumulated quantity Xt is not equal to or less than apredetermined third judgment value X3 (X3 is normally zero) as a resultof the judgment in step S35, the flow returns to step S33 to continuethe first sulfur purge operation control until the accumulated quantityXt becomes not more than the predetermined third judgment value X3. Whenthe accumulated quantity Xt becomes not more than the predeterminedthird judgment value X3 as a result of the judgment in step S35, it isjudged that sulfur purge is completed to stop the first sulfur purgeoperation in step S36 and returns to the normal operation.

In the case of the flow in FIG. 5, the time when the accumulatedquantity Xt becomes not more than the predetermined third judgment valueX3 is assumed as the time when the first sulfur purge operation iscompleted. However, it is also allowed to calculate a sulfur purgeoperation time by collating the accumulated sulfur quantity Xtcalculated in accordance with fuel consumption and a sulfurconcentration of fuel with a sulfur purge operation time map datapreviously input in accordance with the exhaust gas quantity and theexhaust gas temperature Tg at the start of the first sulfur purgeoperation and perform the first sulfur purge operation during thecalculated operation time.

The first sulfur purge operation control in step S33 sets the exhaustgas temperature Tg to a sulfur purge temperature or higher by performingthe rich spike operation when the exhaust gas temperature Tg is higherthan the predetermined temperature Tc (for example, 350° C. to 400° C.)and the exhaust gas temperature Tg may become not less than a sulfurpurge temperature (approx. 400° C.) and reactivates the deteriorateddirect reduction type catalyst 3 by the rich operation preventing thesecondary sulfur poisoning of cerium by making the oxygen concentrationclose to zero to prohibit SO₃ to be produced.

Moreover, the second sulfur purge operation in step S40 is the operationcontrol for forcibly performing the sulfur purge before a proper statefor sulfur purge is realized when, for example, the deterioration of theNOx purging performance becomes problematic by a further progress ofsulfur poisoning or an increase of fuel cost becomes problematic becauseof a frequent regenerating operation for regenerating a catalyst.

The second sulfur purge operation performs the rich spike operationindependently of the operating state to raise the temperature of theexhaust gas so that sulfur is forcibly separated, and the exhaust gastemperature Tg is raised to a sulfur purge temperature or higher by theraised exhaust gas temperature to separate sulfur, and perform thedeteriorated catalyst reactivation.

When the above step S30 or S40 is completed, the flow returns to stepS21 to repeat the operation. Then, when a case of completing the controlflow such as a completion of an engine operation occurs, an interruptionfor the completion of step S50 occurs, the accumulated sulfur quantityXt at the time of completion in step 51, that is, the accumulatedquantity Xt calculated in step S21 or S32 is rewritten in a memory instep S51 and the current flow returns to the NOx purging system controlin FIG. 3 to complete the flow.

Then, when the catalyst regeneration control in FIG. 4 and thedeteriorated catalyst reactivation control in FIG. 5 both return to theNOx purging system control flow in FIG. 3 due to the interruption ofcompletion and further return to a not-illustrated main engine controlflow, and the NOx purging system control flow is also completed at thesame time when the engine control flow is completed.

According to the exhaust gas purging system 10 of the aboveconfiguration and the method of reactivating a deteriorated catalysttherein, it is possible to perform the deteriorated catalystreactivation against sulfur poisoning preventing secondary sulfurpoisoning by using the characteristics that the exhaust gas temperatureTg at the time of reactivating the deteriorated direct reduction typeNOx catalyst 3 is comparatively low at approx. 400° C., therebyperforming the rich spike operation when the exhaust gas temperature Tgreaches a temperature probably exceeding the sulfur purge temperatureTr, that is, the predetermined temperature T1 (e.g. 350° C. to 400° C.)and raising the exhaust gas temperature Tg to the sulfur purgetemperature Tr or higher under low-oxygen conditions.

Moreover, it is also allowed to perform the above control at thetemperature of the direct reduction type NOx catalyst 3 instead ofperforming the control at the exhaust gas temperature Tg. In this case,the predetermined temperature slightly changes.

INDUSTRIAL APPLICABILITY

The present invention provides a NOx purging system using a directreduction type NOx catalyst to purge NOx contained in an exhaust gas anda method of reactivating a deteriorated catalyst therein, in whichexploiting the characteristics that the reactivation of deterioratedcatalyst against sulfur poisoning can be performed at an exhaust gastemperature within the normal operation range, a rich control operationis performed to set the temperature of the direct reduction NOx catalystto a sulfur purge temperature (approx. 400° C.) or higher underlow-oxygen conditions when the exhaust gas temperature becomes not lessthan a predetermined temperature (350° C. to 400° C.) during the normaloperation of an internal combustion engine so that NOx is removedefficiently excluding the influence of sulfur poisoning by the sulfurpurge preventing secondary sulfur poisoning.

Therefore, the present invention can be used for a NOx purging systemprovided with a direct reduction type NOx catalyst in order to purge NOxin the exhaust gas. Thus, the present invention makes it possible toprevent air pollution by efficiently purifying the exhaust gasdischarged from an internal combustion engine of a vehicle and astationary internal combustion engine.

1. A NOx purging system constituted by arranging a direct reduction typeNOx catalyst in an exhaust gas passage in which a catalyst componentreduces NOx to nitrogen and is also oxidized when an oxygenconcentration of the exhaust gas of an engine is high and the catalystcomponent is reduced when the oxygen concentration of the exhaust gaslowers, comprising; a first sulfur purge control means which performs afirst sulfur purge operation for lowering the oxygen concentration ofthe exhaust gas and also raising an exhaust gas temperature to a sulfurpurge temperature or higher when the exhaust gas temperature becomeshigher than a predetermined temperature during the normal operation. 2.The NOx purging system according to claim 1, comprising; a second sulfurpurge control means which performs a second sulfur purge operation forlowering the oxygen concentration of exhaust gas and also raising theexhaust gas temperature to the sulfur purge temperature or higherindependently of the exhaust gas temperature, and a sulfur purge startjudgment means for judging that the present time is a start time of thefirst sulfur purge operation when a deterioration index value indicatinga progress state of catalyst deterioration due to sulfur poisoningranges between a predetermined first reference value and a predeterminedsecond reference value, and judging that the present time is a starttime of the second sulfur purge operation when the deterioration indexvalue exceeds the predetermined second reference value.
 3. A method ofreactivating a deteriorated catalyst for a NOx purging systemconstituted by arranging a direct reduction type NOx catalyst in anexhaust gas passage in which a catalyst component reduces NOx tonitrogen and is also oxidized when an oxygen concentration of theexhaust gas is high and the catalyst component is reduced when theoxygen concentration of the exhaust gas lowers, comprising steps of;carrying a first sulfur purge operation control to perform a firstsulfur purge operation for lowering the oxygen concentration of theexhaust gas and also for raising the exhaust gas temperature to a sulfurpurge temperature or higher when the exhaust gas temperature becomesmore than a predetermined temperature during the normal operation. 4.The method of reactivating the deteriorated catalyst for the NOx purgingsystem according to claim 3, comprising steps of; performing the firstsulfur purge operation when a deterioration index value indicating aprogress state of a catalyst deterioration due to sulfur poisoningranges between a predetermined first reference value and a predeterminedsecond reference value, and performing a second sulfur purge operationfor lowering the oxygen concentration of the exhaust gas and also forraising the exhaust gas temperature to the sulfur purge temperature orhigher when the deterioration index value exceeds the predeterminedsecond reference value independently of the exhaust gas temperature. 5.The method of reactivating the deteriorated catalyst for the NOx purgingsystem according to claim 4, comprising step of; calculating thedeterioration index value indicating a progress state of catalystdeterioration due to sulfur poisoning in accordance with a fuelconsumption and a sulfur concentration of fuel.
 6. The method ofreactivating the deteriorated catalyst for the NOx purging systemaccording to claim 5, wherein; in at least either of the first sulfurpurge operation and the second sulfur purge operation, when thedifference between the sum total of discharged sulfur quantitycalculated by collating a discharged gas quantity and exhaust gastemperature with discharged sulfur quantity map data previously inputand the accumulated sulfur quantity calculated in accordance with thefuel consumption and the sulfur concentration of fuel becomes not morethan a predetermined third reference value, the sulfur purge operationis terminated.
 7. The method of reactivating a deteriorated catalyst fora NOx purging system according to claim 5, wherein; in at least eitherof the first sulfur purge operation and the second sulfur purgeoperation, when a sulfur purge operation time elapses which iscalculated by collating an accumulated sulfur quantity calculated inaccordance with the fuel consumption and the sulfur concentration offuel with sulfur purge operation time map data previously input inaccordance with an exhaust gas quantity and the exhaust gas temperature,the sulfur purge operation is terminated.